Star
Stars are born in the great clouds of gas and dust that are scattered throughout the galaxies. When their nuclear furnaces light up, they start to shine. Some glow feebly; others balze like great celestial beacons. The bigger and brighter a star is, the shorter is its lifetime, and the more spectacular is its death. Stars like the Sun meet quite a peaceful end, shrinking into a super-dense white dwarf as small as a planet. But more massive stars go out with a bang. They blast themselves apart in a mighty supernova explosion and briefly shine nearly as bright as a galaxy. The Birth of a Star When a nearby star explodes, a shock wave travels through the cloud. The cloud begins to shrink and divide into even smaller swirling clouds. As the cloud collapses, energy is released, which causes it to heat up. The center of the cloud, called the protostar, gets hotter and hotter to about 10 million degrees or more until it ignites and a new star is born. Most of the gas in interstellar clouds is hydrogen. And at such high temperatures, the hydrogen atoms start to combine, or fuse together. This fusion reaction produces enormous amounts of energy as light, heat and other radiation. When this happens, the collapsing cloud starts to shine as a star. The outward pressure of the radiation coming from the core of the new star acts against the matter that is collapsing under gravity. Eventually the two balance each other, and the collapse ceases. The star settles down and begins to shine steadily. It takes a star the size of the Sun about 50 million years to reach this state. The hottest stars are blue-white in color and burn their hydrogen fuel very quickly. The Sun, a small yellow star, burns hydrogen more steadily. Proxima Centauri is the closest star to the Sun, burns its gas very slowly and is a cool, red star. The speed at which the stars burn hydrogen determines how long they will live. A Sun-sized star shines steadily for about 10,000 million years, until the hydrogen fuel in its core is used up. The star then begins to collapse again under gravity. The heat triggers off hydrogen fusion in the gassy shell surrounding the core. The shell heats up, causing the star to expand and brighten. But the core continues to shrink and get hotter. Blue giants have a short life, and explode dramatically. The Sun will continue to burn for another 5 billion years. Then it will expand into a large red giant and finally shrink to a white dwarf. Proxima Centauri will remain unchanged for tens of billions of years. The Life Cycle of a Star Stars are born in nebulae. Huge clouds of dust and gas collapse under gravitational forces, forming protostars. These young stars undergo further collapse, forming main sequence stars. Stars expand as they grow old. As the core runs out of hydrogen and then helium, the core contacts and the outer layers expand, cool, and become less bright. This is a red giant or a red super giant (depending on the initial mass of the star). It will eventually collapse and explode. Its fate is determined by the original mass of the star; it will become either a black dwarf, neutron star, or black hole. Category:Out-of-System